Test on Friday!

Lesions of Retinostriate Pathway

• Lesions (usually due to stroke) cause a region of blindness called a scotoma

• Identified using perimetry• note macular sparing

X

Retinocollicular Pathway independently mediates orienting

• Blindsight patients have been shown to posses a surprising range of “residual” visual abilities– better than chance at detection and discrimination of some

visual features such as direction of motion

• These go beyond simple orienting - how can this be?

Retinocollicular Pathway independently mediates orienting

• Recall that the feed-forward sweep is not a single wave of information and that it doesn’t only go through V1

• In particular, MT seems to get very early and direct input – probably from tectopulvinar pathway, but also directly from LGN

Retinocollicular Pathway independently mediates orienting

• Recall that the feed-forward sweep is not a single wave of information and that it doesn’t only go through V1

• In particular, MT seems to get very early and direct input – probably from tectopulvinar pathway, but also directly from LGN

• This input guides behaviour but doesn’t support awareness

Searching for the NCC

• Dorsal/Ventral dichotomy suggests that not all neural processes “cause” consciousness

• Can we find similar dissociations in healthy brains?

• Can we attribute awareness to certain classes of neural events?

Searching for the NCC

• What is needed is a situation in which a perceiver’s state can alternate between “aware” and “unaware” of some information in ways that we can correlate with neural events

• One such situation is called Binocular Rivalry

Rivalrous Images

• A rivalrous image is one that switches between two mutually exclusive percepts

Binocular Rivalry

• What would happen if each eye receives incompatible input?

Left Eye Right Eye

Binocular Rivalry

• What would happen if each eye receives incompatible input?

• The percept is not usually the amalgamation of the two images. Instead the images are often rivalrous.– Percept switches between the two possible images

Binocular Rivalry

• Rivalry does not entail suppression of one eye and dominance of another – it is based on parts of objects:

Left Eye Right Eye

Stimuli:

Percept: Or

Binocular Rivalry

• Percept alternates randomly (not regularly) between dominance and suppression - on the order of seconds– What factors affect dominance and suppression?

Time ->

Binocular Rivalry

• Percept alternates randomly (not regularly) between dominance and suppression - on the order of seconds– What factors affect dominance and suppression?– Several features tend to increase the time one image is

dominant (visible)• Higher contrast• Brighter• Motion

Binocular Rivalry

• Percept alternates randomly (not regularly) between dominance and suppression - on the order of seconds– What factors affect dominance and suppression?– Several features tend to increase the time one image is

dominant (visible)• Higher contrast• Brighter• Motion

• What are the neural correlates of rivalry?

Neural Correlates of Rivalry

• What Brain areas “experience” rivalry?• Clever fMRI experiment by Tong et al. (1998)

– Exploit preferential responses by different regions– Present faces and buildings in alternation

Neural Correlates of Rivalry

• What Brain areas “experience” rivalry?• Clever fMRI experiment by Tong et al. (1998)

– Exploit preferential responses by different regions– Present faces to one eye and buildings to the other

Neural Correlates of Rivalry

• What Brain areas “experience” rivalry?• Apparently activity in areas in ventral pathway

correlates with awareness• But at what stage is rivalry first manifested?• For the answer we need to look to single-cell

recording

Neural Correlates of Rivalry

• Neurophysiology of Rivalry– Monkey is trained to indicate

which of two images it is perceiving (by pressing a lever)

– One stimulus contains features to which a given recorded neuron is “tuned”, the other does not

– What happens to neurons when their preferred stimulus is present but suppressed?

Neural Correlates of Rivalry

• The theory is that Neurons in the LGN mediate rivalry

Neural Correlates of Rivalry

• The theory is that Neurons in the LGN mediate rivalry• NO – cells in LGN respond similarly regardless of

whether their input is suppressed or dominant

Neural Correlates of Rivalry

• V1? V4? V5?• YES – cells in primary and early extra-striate cortex

respond with more action potentials when their preferred stimulus is dominant relative to when it is suppressed

• However,– Changes are small– Cells never stop firing altogether

Neural Correlates of Rivalry

• Inferior Temporal Cortex (Ventral Pathway)?

• YES – cells in IT are strongly correlated with percept

Neural Mechanisms of Consciousness?

• So how far does that get us?

• Not all that far – we still don’t know what is the mechanism that causes consciousness

• But – we do know that it is probably distributed rather than at one locus– We do know that ventral structures seem to play a critical role in visual

awareness

• Thus the question remains: what is special about the activity of networks of neurons that gives rise to consciousness?